The present invention relates to a starting method for induction motors and more particularly to improvements in a starting method for wound-rotor induction motors of the type in which the windings are arranged on the secondary side.
With the recent increase in the atomic power generating equipment, there has been an increasing demand for the automatic frequency controlled operation of hydraulic power plants and the rate of partial-load operation has been increased. As a result, particularly recently an improvement in the efficiency in the partial-load operation has been looked for.
On the other hand, recently there has also been a tendency toward the installation of the hydraulic power generating equipment having large head variations due to limitations to the conditions of location of hydraulic power plants and measures against deterioration of the efficiency due to the head variation have also been looked for.
While various means may be conceived to cope with such deterioration of the efficiency due to the increased head variation, to improve the efficiency through the variable-speed operation of a power generating system has presently been considered to have the greatest realizability. In other words, while, in this type of power generating system, the efficiency of the hydraulic turbine driving the generator will be deteriorated as the head is varied to a lower level, the efficiency of the hydraulic turbine can be prevented from being deteriorated by varying its rotation speed in accordance with the head. Also, the use of such variable-speed power generating system is advantageous in that in the case of a pumped-storage power system the rate of pumping can be adjusted without waste in accordance with the amount of nighttime surplus electric power.
To realize the variable-speed operation of the power generating system, if the generator/motor is a synchronous machine, a thyristor frequency changer is provided between the generator/motor and the electric power system to change the frequency of the generator-motor. In this case, however, the thyristor frequency changer must have a large capacity corresponding to the generation capacity and the manufacturing cost and the power loss during the operation are increased thus making this attempt unpractical.
Then, it is conceivable to use a generator/motor comprising a secondary excitation-type induction motor. The secondary excitation controller of the induction motor has a speed control range with the synchronous speed of the induction motor as the center of the range and the size of the thyristor frequency changer can be decreased by decreasing the speed control range.
An induction motor starting method employing a thyristor frequency changer consisting of a cycloconverter having a controllable output power factor is disclosed in Japanese Patent Unexamined Publication No. 63988/84. This prior art starting method is so designed that the induction motor is started by using the primary-shortcircuiting secondary-excitation variable frequency starting up to 50% of the rated speed and accelerating the motor by the secondary-excitation variable speed operation with the reduced primary voltage between 50 and 100% of the rated speed. This starting method will be described in greater detail with reference to FIG. 1 illustrating a system diagram of an induction generator/motor used in a pumped-storage power plant or the like. In the Figure, numeral 1 designates a pumping hydraulic turbine, and 2 a wound-rotor induction motor for driving the pumping hydraulic turbine 1 (in the case of pumping up). The induction motor 2 includes a primary winding 3 and a secondary winding 4 and the primary winding 3 is connected to the associated circuits through circuit breakers 5, 6 and 7. More particularly, the primary winding 3 is connected to a commercial power source main system 8 through the circuit breaker 5, to the output circuit of an insulating transformer 12 through the circuit breaker 6 and to a short-circuiting circuit 10 through the circuit breaker 7. Numeral 30 designates a filter provided for eliminating the higher harmonic components of the current generated by a cycloconverter. Each of the circuit breakers 5, 6 and 7 may be an oil circuit breaker, magnetic blow-out circuit breaker, vacuum circuit breaker, SF.sub.6 circuit breaker or the like.
The secondary winding 4 of the induction motor 2 is connected to the output side of a cycloconverter 11 including Graetz-connection thyristor circuits 22. The input side of the cycloconverter 11 is connected to the insulating transformer 12. The two reactors of each thyristor circuit 22 are short-circuit preventing reactors. The insulating transformer 12 includes a primary winding 13 connected to the commercial power source main system 8 and secondary windings 14 to 16 connected to the cycloconverter 11.
This conventional starting method will now be described. At the rotation speed of the motor from zero up to 50% of the rated speed, the circuit breaker 7 is closed and the circuit breakers 5 and 6 are opened. In other words, the primary winding 3 of the induction motor 2 is short-circuited via the short-circuiting circuit 10 and the secondary winding 4 is excited at a variable frequency by the cycloconverter 11 thereby starting and accelerating the induction motor 2. When the rotation speed reaches 50% of the rated speed, the circuit breaker 7 is opened and the circuit breaker 6 is closed. Thereafter, the phases of the output currents from the cycloconverter 11 are controlled with respect to the secondary voltage so that the induction motor 2 is brought into a variable speed operation and accelerated up to near 100% of the rated speed. After the rotation speed of the induction motor 2 has reached 100% of the rated speed, the circuit breaker 6 is opened and the circuit breaker 5 is closed thus bringing the induction motor 2 into the ordinary secondary-excitation variable speed operation. While this conventional starting method utilizes the exciting cycloconverter 11 for starting purposes thus effecting the starting with a sufficiently large torque without using any special device, the variable frequency range of the cycloconverter 11 is limited and generally the frequency can be controlled over a range from 0 to about 50% of the system frequency. As a result, the circuit breakers 6 and 7 must be closed and opened, respectively, at around 50% of the rated speed so as to effect the change-over from the variable frequency operation to the secondary-excitation variable speed operation. These switching operations of the circuit breakers 6 and 7 cause considerable shock such as voltage variation in the main system. Also, since the circuit breakers 5 and 6 are connected to the power source, in order to close them, it is necessary to control so that the closing is effected when the frequency, level and phase of the voltage of the motor are the same with those of the power source.